Piston rings are a critical component of an internal combustion engine. The engine includes at least one cylinder and piston. Piston rings are metallic seals disposed between the cylinder wall and the piston to seal the combustion chamber from the crankcase and facilitate heat transfer from the piston to the cylinder. Other functions of piston rings are to prevent the oil not needed for lubrication from passing from the crankcase to the combustion chamber and to provide a uniform oil film on the cylinder bore surface. To achieve this, the piston rings must remain in contact with the cylinder and the piston. Radial contact is generally achieved by means of the inherent spring force of the piston ring. Piston rings are also employed as metallic seals for rotating shafts and are used both as contracting and expanding seals.
Today, piston rings are typically manufactured in one of two ways. In one method, the piston rings are cast as individual rings in a noncircular shape. Such rings are then typically machined to the required shape by means of double cam turning, a process in which the ring blank, already axially ground, is cam turned simultaneously on the inside and outside diameters. After a segment equivalent to the free gap is cut from the piston ring, it assumes the free shape that will give it the required radial pressure distribution when fitted into the cylinder. Once inside the cylinder, the piston ring exerts the predefined radial pressure against the cylinder wall. Besides using double cam turning, ring blanks may also be shaped by machining the inside and outside diameters separately. This involves cam turning the outside diameter of the noncircular blank and machining the inside diameter with the piston ring in the compressed state. The free gap is cut out in a step between the outer diameter and inner diameter machining.
According to a different method, steel piston rings are made from a profiled wire. The rings are first coiled into a circular shape and then the gap is cut out. The necessary shape is obtained using a heat treatment process in which the rings are mounted onto an arbor appropriately designed to impart the required radial pressure distribution. Profiling of the outer diameter is carried out, depending on the piston ring design, on automatic outer diameter lathes or profile grinding machines using profile cutting tools.
The problem with manufacturing piston rings according to the methods described above is that the piston rings are produced with residual tangential tension. Production of piston rings with residual tangential tension is problematic because such piston rings have a tendency to twist or warp. Such ring twist or warp may lead to excessive oil consumption and blow-by, a condition where combustion gasses escape from the combustion chamber by passing along the piston between the piston rings and the cylinder wall. Accordingly, piston rings with residual tangential tension may adversely affect the efficiency, the performance, the emissions, and/or the reliability of the engine.
Methods aimed at producing piston rings with reduced tendency to twist or warp have been developed. One such method includes the step of heat treating a stock bar made of cast iron at a high temperature, for example 1100° Fahrenheit (593.33° Celsius), to remove foundry strains and hard spots. After the stock bar is heat treated, piston ring blanks are cut from the stock bar. Once the piston ring blanks are detached from the stock bar, the method continues with the steps of conventional machining and finishing the piston ring blank to final outer and inner diameters. The last step is cutting a final free gap into the piston ring. Although this method produces rings having less of a tendency to twist or warp, significant tangential tension still remains in the piston rings.
What is needed is a method of manufacturing piston rings wherein only a negligible amount of residual tangential tension remains in the finished products. Without residual tension, the rings will not have a tendency to twist or warp. Residual tension is especially a concern with piston rings with unconventional cross-sections, such as dyke-type piston rings which have an L-shaped cross-section.
Dykes type piston rings allow for better sealing at higher engine speeds and combustion pressures. However, the asymmetrical shape of the dykes type piston ring results in a piston ring that is more prone to twist or warp than conventional piston rings. Accordingly, there is a need for piston rings, particularly dykes-type piston rings with negligible tangential tension.